An IGBT module including an IGBT and a free wheeling diode (hereinafter, which is abbreviated to an FWD), which are power semiconductor devices, is applied to a power conversion device, such as an inverter or a DC (Direct Current) chopper circuit.
In general, it is necessary to detect an output current in order to control the power conversion circuit. In general, the following two methods are mainly used to detect the output current:
(1) A method using a current detector, such as a current transformer or a DC-CT (Direct Current-Current Transformer); and
(2) A method using a current detecting resistor (which is called a shunt resistor).
However, the current detection methods according to the related art have the following problems.
In the method using the current transformer, since a Hall sensor or a core is used, the device is expensive. In addition, since the core is used, the size of the detector increases and there are restrictions in reducing the size of the power conversion device.
In the method using the shunt resistor, since power loss occurs due to the resistor, the power conversion efficiency of the power conversion device is reduced. In addition, the size of the resistor increases in order to allow large loss and there are restrictions in reducing the size of the power conversion device.
In some cases, the power semiconductor device has a current detecting function which is called a “power semiconductor device with a sensing function”. In this case, the power semiconductor device is divided into a main region and a sense region, a sense current flowing to the sense region can be measured, a main current flowing to the main region can be estimated and detected on the basis of a known sense ratio. The current sense ratio is determined by the area ratio W of the main region and the sense region. In general, the area of the main region is set to be several thousand times that of the sense region. For example, JP 2000-134955 A (FIG. 1 and FIG. 18) and JP 2003-274667 A (FIG. 1, FIG. 4, and FIG. 7) disclose a technique for detecting a current using the sense region. In addition, JP 10-32476 A (FIG. 8 and FIG. 9) discloses an application of a power semiconductor device with a sensing function in which the current flowing to the main region is presumed to be an overcurrent when the current flowing to the sense region is greater than a predetermined value.
In addition, a method has been known in which, since a small amount of current flows to the sense region, a shunt resistor is connected to a sense terminal and a sense current is measured. In this case, it is possible to solve the problem of power loss due to the resistor. However, the detection accuracy of the current is reduced. The detection accuracy is reduced for the following two reasons. First, (a) ideally, the ratio of the main current and the sense current is determined by the area ratio of the main region and the sense region. However, an error occurs in the current sense ratio according to the structure or layout of the device. That is, an error caused by the characteristic difference between the main region and the sense region is the first point. Then, (b) the sense current is significantly less than the main current and a voltage drop occurs due to sense resistance. Therefore, in an example of the structure of a “sense IGBT” including a shunt resistor 703 according to the related art illustrated in FIG. 1, a voltage Vce1 between the collector and the emitter of a main IGBT 701 is different from a voltage Vce2 between the collector and the emitter of a sense IGBT 702 and an error occurs in the current sense ratio with respect to the area ratio of the main region and the sense region. That is, an error which is caused by a voltage drop due to the shunt resistor is the second point. In addition, JP 2006-271098 A (FIG. 1) discloses a reduction in the sense ratio due to sense resistance. In JP 2006-271098 A (FIG. 1), the sense resistance is minimized to reduce the influence of the sense resistance.
The current sense ratio is changed by a variation in the temperature of the power semiconductor device. Ideally, the main current and the sense current have the same temperature characteristics. However, as described above, an error occurs in the current sense ratio according to, for example, the structure of the device. Similarly, the temperature coefficient of the main current is not completely equal to that of the sense current and an error occurs in the current sense ratio according to the temperature. When the ambient temperature of the power semiconductor device with a sensing function which detects a current is changed, the current detection accuracy is reduced. For example, this is clarified by FIG. 8 illustrating a graph of the measurement result of a variation in the current sense ratio due to the temperature of a general power semiconductor device, which will be described below. The variation in the current sense ratio due to the temperature is disclosed in the following Patent Document.
JP 2005-50913 A (FIG. 5, FIG. 6, and FIG. 8) discloses a method in which, since resistance temperature coefficients are different in the plane of the device, the cross-sectional structures of the main region and the sense region are individually adjusted or the layout of the main region and the sense region is adjusted to reduce the temperature dependence of the current sense ratio. However, in the method of reducing the temperature dependence of the current sense ratio disclosed in JP 2005-50913 A (FIG. 5, FIG. 6, and FIG. 8), it is predicted that flexibility in the design of the power semiconductor device will be reduced.
JP 2010-199279 A (FIG. 3, FIG. 4, and FIG. 7) discloses a method which changes the bonding position of a Kelvin terminal that extracts a sense current from the main region in order to measure the sense current to increase a resistance component on a bonding pad and offsets the temperature characteristics of a sense element with the temperature characteristics of pad resistance to reduce the temperature dependence of the current sense ratio. However, the method of reducing the temperature dependence of the current sense ratio disclosed in JP 2010-199279 A (FIG. 3, FIG. 4, and FIG. 7) is just a method of providing qualitative improvement and has the problem that the effect is greatly changed depending on a variation in the resistance of the pad or the characteristics of the sense element.
JP 2006-271098 A (FIG. 1) discloses a method which detects the temperature and performs an operation for the sense current output result according to the temperature to reduce the temperature dependence of the current sense ratio. However, the method of reducing the temperature dependence of the current sense ratio disclosed in JP 2006-271098 A uses the shunt resistor. When the shunt resistor is used, power loss occurs due to the resistor, as described above. Therefore, the power conversion efficiency of the power conversion device is reduced. In addition, a microcomputer constantly performs a digital operation for the detected sense current and outputs the operation result. Therefore, a high-speed ADC (Analog-Digital Converter) and a high-speed calculating unit are needed.